Progress in our understanding the neuroanatomical bases and the neurophysiological mechanisms that underlie human language learning and expression and sexually related language=-dependent tasks depends on the use of appropriate animal models. The best such model currently available to us is the songbird. In songbirds, song is a learned behavior which is generally sexually dimorphic and is controlled by a set of dedicated and interconnected brain regions collectively called the vocal control system. This system retains an unusual degree of plasticity in adulthood in that in some regions, new neurons are added and form functional connections or are eliminated throughout life. Sex steroids play a pivotal role in this plasticity as well as in vocal behavior expression. In other experimental models, effects of sex steroids result from changes in opioid synthesis and release, and in mammals, opioids induce neuronal actions that in many respects resemble those of steroids. The avian vocal control system contains opioid peptides, and as demonstrated by the P.I.'s investigations, also opioid receptors. Whether behavioral and neuroanatomical actions of s ex steroids on this system are opioid-mediated has, however, not been examined. To address this question, the P.I.will use a seasonal species to establish whether seasonal change in plasma sex steroid concentrations correlate with alterations in vocal control region opioid expression. He will take advantage of the fact that males, but not females, of his study species normally sing to determine whether vocal control region opioid expression differs between sexes, and he will experimentally manipulate circulating sex steroid concentrations to test whether this expression in steroid-dependent. He will additionally assess the effects of opioid receptor blockade on song rate and structure, and he will investigate whether these effects are centrally mediated and concur with changes in vocal control region morphological characteristics. The proposed research will determine, for the first time, whether actions of sex steroids on vocal behavior and its neuroanatomical substrate involve opioids. It will include training of the P.I. in two new research directions (quantitative light microscopy morphometry; computerized sound analysis) that are directly relevant to studies on human speech, language, and voice.